1. Field of the Invention
The field of the invention is that of tactile or “touchscreen” surfaces with capacitive detection and more particularly so-called “multitouch” tactile surfaces allowing the detection of two simultaneous presses. This function is essential for carrying out for example image “zooms” or rotations. This invention can apply in various uses but it is particularly well suited to the constraints of the aeronautical field and aircraft instrument panels.
2. Description of the Prior Art
So-called “projected” capacitive detection consists in producing a detection matrix arranged so as to detect the local variations in capacitance which are introduced by the proximity of the fingers of the user or of any other conducting pointing object. So-called projected capacitive technology comes in two main variants, namely:                “Self capacitive” detection which consists in reading the rows and then the columns of the array of touchkeys of the matrix:        So-called “Mutual capacitive” detection consisting in reading each intersection of the array of touchkeys of the matrix;        
“Mutual capacitive” technology requires the reading of the whole of the pad. Thus, if the matrix comprises N rows and M columns, it is necessary to carry out N×M acquisitions, making it problematic to produce pads of large size, of high resolution and with low response time. Moreover, the capacitance to be measured under “Mutual capacitance” is lower than that obtained under “Self capacitive”, thus making it problematic for the user to use gloves.
The advantage of “Self capacitive” detection is that, for the above pad, the system requires only N+M acquisitions to carry out a reading of the matrix. FIG. 1 illustrates this principle. In this FIG. 1, a first finger presses at the level of a first intersection of column CI and of row LJ and a second finger presses at the level of a second intersection of column CK and of row LL. The output voltages VOUT of the rows and columns exhibit easily identifiable drops in level. The measurements of the voltages around each drop in level make it possible to identify precisely the rows and columns invoked.
However, this latter technique exhibits a drawback. It is not always simple to ascribe the rows and the columns detected to the correct intersections actually touched by the user's fingers. Possible intersections, but which are not actually touched, are generally called “ghosts”. To counter this difficulty, a technique consists in carrying out a scan of the matrix at two different acquisition frequencies. This technique is described in the publication “Eliminating Ghost Touches on a Self-Capacitive Touch-Screen” published in “SID 2012 DIGEST” of June 2012.
To properly understand this technique, it is necessary to use electronic models to represent a capacitive matrix device. There exists a simplified model for describing electrically a capacitive tactile device comprising a matrix of electrodes which is composed of conducting rows and of columns. It consists of a representation of a press where the operator's finger is coupled capacitively with the matrix by projecting the surface of his finger on the pad. This surface covers at least two electrodes, a first row-wise and a second column-wise. It is then considered that the operator adds a capacitance Cd between the earth and at least the row or the column concerned. But this model remains local and does not take account of the environment of the measurement.
FIG. 3 represents a more elaborate model of a capacitive matrix device. Each row is in fact connected to a measurement and/or power supply device through analogue switches. These switches afford a coupling capacitance Cm with respect to the ground and exhibit an electrical resistance Rm causing an attenuation of the measured signal.
Moreover, each row consisting of a transparent material of ITO (Indium Tin Oxide) type which exhibits a certain resistance between the point of power supply and the point of contact of the finger, this resistance being all the higher the further away the finger is from the connection point. If Rt is the resistance of a touchkey and of its connection to the next, then the resistance between the press on column n and the edge of the matrix is n·Rt.
Furthermore, the array of rows and columns is mutually coupled. Indeed, there exists a capacitance Cp at each crossover of tracks, each row being cut by n column and furthermore, the rows or column are coupled with their neighbours. This coupling is represented in FIG. 3 by a capacitance CIc. Finally, there also exist coupling capacitances between the touchpad, its connector arrangement and the mechanical items constituting the device, as well as a mutual coupling between the various tracks linking the rows and the columns to the electronic measurement device.
Consequently, the acquisition of a capacitive touchpad may not be reduced to the acquisition of a simple capacitance projected by an operator. It is the result of this projection on a multipole complex hardware component composed of an association of resistors and of capacitors that are mutually interconnected.
The “Dual-frequency self capacitive” device utilizes this complexity. As seen in FIG. 3, in the absence of any object in the neighbourhood of the matrix, each row Li is linked to an AC voltage power supply across an injection capacitance Ci and to a reading buffer which possesses an input impedance consisting of a stray coupling capacitance relative to the ground Cm and an input resistance Rm. This row Li possesses a lineal resistance and is coupled capacitively to each column crossover.
When a finger is placed on a precise point of the row Li, it projects a capacitance onto the portion of the row considered. The tactile devices according to the prior art measure only this projected capacitance. This simple measurement does not make it possible to ascertain the position of the press on the row, this information not being conveyed by the value of the projected capacitance.
The heart of the device is not to consider simply the added capacitance, but its effect on the complex model constituted by the entire row. In particular, if the resistance Ril of row Li of length l is considered, then there exists a resistance Ria between an end of the row and the contact point. The resistance Ria is less than Ril. This resistance value modifies the output signal VOUT. This signal VOUT equals:
VOUT=Z·VIN with VIN: periodic input signal of frequency F and Z: impedance of the row which equals:
Z=A+Bj The terms A and B being functions of the capacitances Cm, Ci and Cd and the resistances Rm and Ria.
The topology of the model is akin to first order to an RC network or the resistance Ria associated with the capacitance Cd constitutes a first-order low-pass filter. FIG. 4 represents, as a function of the applied frequency, the variation of the output signal of a row for three different positions of pressing, the first curve C1 for a press situated at a row edge, the second C2 for a press in the middle of a row, the third curve C3 for a press at the end of a row. The scale of FIG. 4 is logarithmic on both axes. There then exists, as seen in FIG. 4, a frequency FMIN such that the variations of Ria cause a minimum variation of VOUT whatever the position of the press. Note, for example, that in the absence of any press, a low value working frequency (x-axis) causes the reception voltages (VOUT) to have minimal variations (y-axis). Conversely, there exists a frequency FMAX, which also includes a high value discrimination frequency, such that the variations of Ria cause a significant attenuation (also referred to as distinguishable variations) of VOUT as a function of the position of the press. Thus, at this frequency FMAX, by measuring this attenuation, it is then easy to ascertain the value of the resistance Ria and consequently, to determine the position of the contact point on the row.
This measurement is not necessarily very precise. It is however sufficient to determine the actual position of two simultaneous presses. Knowing, even approximately, through the double measurement at two different frequencies, the positions of the presses, the indeterminacy between the pair of actual presses and the pair of ghost presses or “ghosts” which corresponds thereto is lifted.
There still exist however certain configurations that may engender either an uncertainty in the position of the user's fingers or a positioning error. One of these configurations occurs when two fingers touch two neighbour rows or columns. This configuration is illustrated in FIG. 2. In this figure, the first finger presses at the level of a first intersection of column CI and of row LI and a second finger presses at the level of a second intersection of column CJ and of row LI+1. The output voltage VOUT of the columns exhibits two easily identifiable spikes making it possible to determine that columns CI and Cj have been pressed. On the other hand, the output voltage VOUT of the rows exhibits a notch-gated form not making it possible to clearly identify the rows concerned, even using barycentric calculations of voltage.